This invention relates to a primary lithium electrochemical cell and a method of manufacturing a primary lithium electrochemical cell.
A battery includes one or more galvanic cells (i.e., cells that produce a direct current of electricity) in a finished package. Cells of this type generally contain two electrodes separated by a medium capable of transporting ions, called an electrolyte. Typical electrolytes include liquid organic electrolytes or a polymeric electrolytes. The cell produces electricity from chemical reactions through oxidation at one electrode, commonly referred to as the negative electrode or anode, and reduction at the other electrode, commonly referred to as the positive electrode or cathode. Completion of an electrically conducting circuit including the negative and positive electrodes allows ion transport across the cell and discharges the battery. A primary battery is intended to be discharged to exhaustion once, and then discarded. A rechargable battery can be discharged and recharged multiple times.
An example of a primary battery is a primary lithium cell. A lithium electrochemical cell is a galvanic cell using lithium, a lithium alloy or other lithium containing materials as one electrode in the cell. The other electrode of the cell can include, for example, a transition metal oxide, such as gamma-manganese dioxide (xcex3-manganese dioxide or xcex3-MnO2) or transition metal sulfide such as iron disulfide. The metal oxide or sulfide used in the electrode can be processed prior to use in a lithium battery. Generally, xcex3-manganese dioxide can be prepared by chemical methods or electrochemical methods. The resulting materials are known as chemically produced xcex3-manganese dioxide (CMD) and electrochemically produced (e.g., electrolytic or electrodeposited) xcex3-manganese dioxide (EMD), respectively.
A primary electrochemical cell includes a cathode including lambda-manganese dioxide (xcex3-MnO2) having a spinel-related crystal structure. The anode can include lithium metal or a lithium alloy, such as lithium aluminum alloy.
In one aspect, a primary lithium electrochemical cell includes a cathode including lambda-manganese dioxide, an anode including lithium, a separator between the anode and the cathode, and an electrolyte contacting the cathode, the anode and the separator. The cell has an average closed circuit voltage (CCV) between 3.8 and 4.1V and a specific discharge capacity to a 3V cutoff of greater than about 130 mAh/g at a nominal discharge rate of 1 mA/cm2. The cell can have a 3V cutoff of greater than 135 mAh/g or 140 mAh/g or greater at a nominal discharge rate of 0.4 mA/cm2.
The lambda-manganese dioxide can be heated to a temperature of less than 150xc2x0 C., or 120xc2x0 C. or less, during processing and cathode fabrication.
In another aspect, a method of preparing lambda-manganese dioxide includes contacting water with a compound having the general formula Li1+xMn2xe2x88x92xO4, wherein x is from xe2x88x920.02 to +0.02, or xe2x88x920.005 to +0.005, adding an acid to the mixture of water and the compound until the water has a pH of 1 or less, preferably between 0.5 and 1, separating a solid product from the water and acid, and drying the solid at a temperature of 150xc2x0 C. or less to obtain the lambda-manganese dioxide. The method can include washing the solid separated from the water and acid with water until the washings have a pH of between 6 and 7.
The compound can have a BET surface area of between 1 and 10 m2/g, or greater than 4 m2/g or greater than 8 m2/g, a total pore volume of between 0.02 and 0.2, or 0.05 and 0.15, cubic centimeters per gram, or an average pore size of between 100 and 300 xc3x85.
The solid can be dried at a temperature between 20xc2x0 C. and 120xc2x0 C., 30xc2x0 C. and 90xc2x0 C., or between 50xc2x0 C. and 70xc2x0 C. A vacuum optionally can be applied during drying.
Contacting water and the compound includes forming a slurry. The slurry can be maintained at a temperature below 50xc2x0 C. or between about 10xc2x0 C. and 50xc2x0 C., or about 15xc2x0 C. to 30xc2x0 C. The acid can be sulfuric acid, nitric acid, perchloric acid, hydrochloric acid, toluenesulfonic acid or trifluoromethylsulfonic acid. The acid solution can have a concentration between 1 and 8 molar. The temperature of the slurry can be held substantially constant during the addition of the acid.
In another aspect, a method of manufacturing a primary electrochemical cell includes providing a positive electrode including lambda-manganese oxide and forming a cell including the positive electrode and a negative electrode including lithium. The cell has a closed circuit voltage between 3.8V and 4.1V and a specific discharge capacity to a 3V cutoff of greater than about 130 mAh/g at a nominal discharge rate of 1 mA/cm2. Providing the electrode can include preparing lambda-manganese dioxide by a method including contacting water with a compound of the formula Li1+xMn2xe2x88x92xO4, wherein x is from xe2x88x920.02 to +0.02, adding an acid to the water and compound until the water has a pH of 1 or less, separating a solid from the water and acid, and drying the solid at a temperature of 150xc2x0 C. or below to obtain the lambda-manganese dioxide. The electrode can be fabricated by mixing the lambda-manganese dioxide with a conductive additive and an optional binder.
A primary lithium electrochemical cell including a cathode containing xcex3-MnO2 can have an average closed circuit voltage of between 3.8V and 4.1V, a specific discharge capacity to a 3V cutoff of greater than 135 mAh/g at a discharge rate of 1 mA/cm2, good high-rate performance, and adequate capacity retention when stored. A closed circuit voltage of about 4V can provide desirable voltage compatibility with lithium-ion secondary cells having cathodes containing LiCoO2, LiNiO2 or solid solutions thereof (i.e. LiCoxNi1xe2x88x92xO2, wherein 0 less than x less than 1). A specific single cycle capacity of greater than 135 mAh/g can provide greater capacity than the average single cycle capacity for a typical lithium-ion secondary cell having a cathode containing LiCoO2, LiNiO2 or solid solutions thereof. Adequate capacity retention when stored can be especially important because in a primary electrochemical cell any loss of capacity cannot be recovered through recharging. A primary lithium electrochemical cell having a cathode including xcex3-MnO2 can have a higher total energy density than a primary lithium electrochemical cell having a cathode including heat-treated xcex3/xcex2-MnO2 and having an average closed circuit voltage of about 2.8V.
The physical and chemical properties of a lithium manganese oxide spinel powder (LiMn2O4) used as a precursor for the xcex3-MnO2, especially the chemical stoichiometry and the particle microstructure, can dramatically influence the 4V discharge capacity and the thermal stability of the resulting xcex3-MnO2 product. A high-capacity xcex3-MnO2 can be produced by substantially completely removing of lithium from the spinel lattice of a nominally stoichiometric precursor spinel, for example, by treating the spinel with acid to a pH value of less than 2. By avoiding heat treatment of the xcex3-MnO2 powder above 150xc2x0 C., for example in the range of 80xc2x0 C. to 120xc2x0 C., decomposition of the xcex3-MnO2 can be reduced or avoided, leading to a specific capacity for a low-rate discharge of greater than 130 mAh/g to a 3V cutoff, but less than the theoretical 4V capacity of about 154 mAh/g.
Other features and advantages of the invention will be apparent from the description of the preferred embodiments and from the claims.